Heparin-stabilized gold nanoparticles embedded in graphene for the electrochemical determination of esculetin.

A conductive nanocomposite consisting of heparin-stabilized gold nanoparticles embedded in graphene was prepared and characterized to develop an electrochemical sensor for the determination of esculetin in tea and jam samples. The gold nanoparticles were characterized by spectroscopic and microscopic techniques. The different proportions of graphene in the nanocomposite were evaluated and characterized by electrochemical practices. The heterostructure material on the glassy carbon electrode with esculetin showed π-π stacking interactions with an adsorption-controlled process. The voltammetric profile of esculetin using the proposed nanomaterial presented oxidation and reduction peaks at +0.61 and +0.58 V vs. Ag/AgCl, respectively, facilitating the electron transfer with esculetin through the transfer of two moles of protons and two moles of electrons per mole of esculetin. Using optimized conditions and square wave voltammetry, the calibration curve was obtained with two linear ranges, from 0.1 to 20.5 µmol L-1, with a detection limit of 43.0 nmol L-1. The electrochemical sensor showed satisfactory results for repeatability and stability, although interferences were observed in the presence of high concentrations of ascorbic acid or quercetin. The sensor was successfully applied in the determination of esculetin in samples of mulberry jam, white mulberry leaf tea, and white mulberry powder tea, presenting adequate recovery ranges. This directive provides valuable insights for the development of novel electrochemical sensors using heparin-based conductive nanomaterials with improved sensitivity and sensibility.

Determination of paracetamol using a sensor based on green synthesis of silver nanoparticles in plant extract.

The biosynthesis of nanometals using a plant extract is simple, efficient, fast, cost-effective and eco-friendly. In this study, a pine nut extract (Araucaria angustifolia) was obtained and used as a reducing and stabilizing agent in the synthesis of silver nanoparticles. An electrochemical sensor based on the silver nanoparticles obtained and exfoliated graphite nanoplatelets applied to a glassy carbon electrode was developed for the determination of paracetamol. To optimize the synthesis of the silver nanoparticles, important factors such as temperature, extract:water ratio, silver nitrate concentration and extract stability time were studied. The factors influencing the performance of the sensor were studied in detail and the results demonstrated good repeatability and electrode-to-electrode repeatability (relative standard deviations of 1.8 and 4.0 %, respectively). Under optimized conditions, there was a linear response to paracetamol concentrations of 4.98 × 10-6 to 3.38 × 10-5 mol L-1, with a detection limit of 8.50 × 10-8 mol L-1. No reports on the biosynthesis of AgNPs using Araucaria angustifolia could be found in the literature. The sensor developed showed good stability and was used successfully for the quantification of paracetamol in pharmaceutical products.

Self-assembled monolayer of nickel(II) complex and thiol on gold electrode for the determination of catechin.

Self-assembled monolayers of a nickel(II) complex and 3-mercaptopropionic acid on a gold electrode were obtained for determination of catechin by square wave voltammetry. The complex [Ni(II)L] with L=[N-(methyl)-N'-(2-pyridylmethyl)-N,N'-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-1,3-propanediamine[nickel(II)] was synthesized and characterized by (1)H NMR, IR, and electronic spectroscopies and electrochemical methods. The optimized conditions obtained for the electrodes were 0.1 mol L(-1) phosphate buffer solution (pH 7.0), frequency of 80.0 Hz, pulse amplitude of 60.0 mV and scan increment of 10.0 mV. Under these optimum conditions, the resultant peak current on square wave voltammograms increases linearly with the concentration of catechin in the range of 3.31 x 10(-6) to 2.53 x 10(-5)mol L(-1) with detection limits of 8.26 x 10(-7)mol L(-1). The relative standard deviation for a solution containing 1.61 x 10(-5)mol L(-1) catechin solution was 2.45% for eight successive assays. The lifetime of the Ni(II) complex-SAM-Au electrode was investigated through testing every day over 4 weeks. The results showed apparent loss of activity after 20 days. The results obtained for catechin in green tea samples using the proposed sensor and those obtained by electrophoresis are in agreement at the 95% confidence level.

Biosensor based on laccase and an ionic liquid for determination of rosmarinic acid in plant extracts.

Novel biosensors based on laccase from Aspergillus oryzae and the ionic liquids (ILs) 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIPF(6)) and 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIBF(4)) were constructed for determination of rosmarinic acid by square-wave voltammetry. The laccase catalyzes the oxidation of rosmarinic acid to the corresponding o-quinone, which is electrochemically reduced back to rosmarinic acid at +0.2V vs. Ag/AgCl. The biosensor based on BMIPF(6) showed a better performance than that based on BMIBF(4). The best performance was obtained with 50:20:15:15% (w/w/w/w) of the graphite powder:laccase:Nujol:BMIPF(6) composition in 0.1mol L(-1) acetate buffer solution (pH 5.0). The rosmarinic acid concentration was linear in the range of 9.99 x 10(-7) to 6.54 x 10(-5)mol L(-1) (r=0.9996) with a detection limit of 1.88 x 10(-7)mol L(-1). The recovery study for rosmarinic acid in plant extract samples gave values from 96.1 to 105.0% and the concentrations determined were in agreement with those obtained using capillary electrophoresis at the 95% confidence level. The BMIPF(6)-biosensor demonstrated long-term stability (300 days; 920 determinations) and reproducibility, with a relative standard deviation of 0.56%.

Determination of chlorogenic acid in coffee using a biomimetic sensor based on a new tetranuclear copper(II) complex.

A new tetranuclear copper(II) complex which mimics the active site of catechol oxidase was synthesized and characterized by IR, CHN, electronic spectroscopic and (1)H NMR methods. The title complex [Cu(2)(mu-OH)(bpbpmp-NO(2))](2)[ClO(4)](2) was employed in the construction of a novel biomimetic sensor and used in the determination of chlorogenic acid by square wave voltammetry. The performance and optimization of the resulting biomimetic sensor were studied in detail. The best response of this sensor was obtained for 75:15:10% (w/w/w) ratio of the graphite powder:nujol:Cu(II) complex, 0.1 mol L(-1) phosphate buffer solution (pH 7.0), with frequency, pulse amplitude, and scan increment at 30 Hz, 100 mV, and 3.0 mV, respectively. The chlorogenic acid concentration was linear in the range of 5.0x10(-6) to 1.45x10(-4)mol L(-1) (r=0.9985) with a detection limit of 8.0x10(-7)mol L(-1). This biomimetic sensor demonstrated long-term stability (250 days; 640 determinations) and reproducibility, with a relative standard deviation of 10.0%. The recovery study of chlorogenic acid in coffee samples gave values from 93.2% to 106.1% and the concentrations determined showed good agreement when compared with those obtained using capillary electrophoresis at the 95% confidence level.

Rosmarinic acid determination using biomimetic sensor based on purple acid phosphatase mimetic.

A new heterodinuclear Fe(III)Zn(II) complex which mimics the active site of the hydrolytic enzyme red kidney bean purple acid phosphatase was synthesized and characterized by IR, CHN and X-ray crystallographic analyses. This complex, [Fe(III)Zn(II)(mu-OH)bpbpmp-CH(3)](ClO(4))(2), containing the ligand (H(2)bpbpmp-CH(3) = {2-[bis(2-pyridylmethyl)aminomethyl]-6-[(2-hydroxy-5-methylbenzyl) (2-pyridyl-methyl) aminomethyl]-4-methyl-phenol}) was employed in the construction of a biomimetic sensor and used in the determination of rosmarinic acid in plant extract samples. The response parameters and optimization of the biomimetic sensor design were evaluated. The best performance of this sensor was obtained for 75:15:10% (w/w/w) of the graphite powder:nujol:Fe(III)Zn(II) complex, 0.1 mol L(-1) phosphate buffer solution (pH 7.5), 1.19x10(-4) mol L(-1) hydrogen peroxide with frequency, pulse amplitude, and scan increment at 30 Hz, 100 mV, and 0.6 mV, respectively. The rosmarinic acid concentration was linear in the range of 2.98x10(-5) to 3.83x10(-4) mol L(-1) (r=0.9991) with a detection limit of 2.30x10(-6) mol L(-1). This biomimetic sensor demonstrated long-term stability (300 days; 900 determinations) and reproducibility, with a relative standard deviation of 12.0%. The recovery study of rosmarinic acid in plant extract samples gave values from 90.3 to 98.3% and the concentrations determined showed agreement when compared with those obtained using capillary electrophoresis at the 95% confidence level.

Development of a biosensor based on gilo peroxidase immobilized on chitosan chemically crosslinked with epichlorohydrin for determination of rutin.

A new reagentless biosensor for the square-wave voltammetric determination of rutin in pharmaceutical formulations was developed by immobilization of gilo (Solanum gilo) crude extract in chitosan matrix. The gilo tissue acts as a source of peroxidase. The highest biosensor performance was obtained after immobilization of the peroxidase in chemically crosslinked chitosan with epichlorohydrin and glutaraldehyde that was incorporated in a carbon paste electrode. In the presence of hydrogen peroxide this enzyme catalyses the oxidation of rutin to quinone and the electrochemical reduction of the product was obtained at a fixed potential of +124 mV versus Ag/AgCl (3.0 M KCl). The performance and factors influencing the resulting biosensor were studied in detail. The bioelectrode exhibited a linear response for rutin concentrations from 3.4x10(-7) to 7.2x10(-6) M (r=0.9998) and the recovery of rutin from the samples ranged from 96.2 to 102.4%. The detection and quantification limits were 2.0x10(-8) and 6.3x10(-8) M, respectively. The relative standard deviation was less than 1.0% for solutions containing 3.4x10(-7) to 7.2x10(-6) M rutin in 0.1 M phosphate buffer solution at pH 7.0 (n=10). The lifetime of this biosensor was 8 months (at least 500 determinations).

A zucchini-peroxidase biosensor applied to dopamine determination.

A biosensor modified with peroxidase from crude extract of zucchini (Cucurbita pepo) was developed for the determination of dopamine in pharmaceutical samples. This enzyme catalyses the oxidation of dopamine to dopaminequinone, in presence of hydrogen peroxide, which the electrochemical reduction can be followed at a peak potential of -0.02 V. The recovery of dopamine from the samples ranged from 94.8% to 106% and a rectilinear analytical curve for dopamine concentration from 5.0 x 10(-4) to 3.0 x 10(-3) mol L-1 (r=0.9982) was obtained. The detection limit was 2.6x10(-5) mol L-1 and the relative standard deviation was less than 1.2% for 7.9 x 10(-4) mol L-1 dopamine in 0.1 mol L-1 phosphate buffer solution at pH 6.0 (n=10).